1. Field of the Invention
The present invention relates to a machine tool such as a machining center, which includes a rotary table, and more particularly, to a machine tool having a function of automatically and optimally setting an unclamp timer in a clamping mechanism of a rotary table.
2. Description of the Related Art
A rotary table such as an index table has been widely used in a machine tool such as a machining center. In particular, in various types of dividing mechanisms (for example, a dividing mechanism using a servo motor of a turret tool post) for performing a dividing operation of a workpiece in the machine tool, a clamping mechanism has been used such that a location is fixed after a turning and dividing operation. In the clamping mechanism, not only a motor or control but also the clamping mechanism is an important factor to perform a highly reliable dividing operation. When another turning and dividing operation is performed again after performing a turning and dividing operation in the clamping mechanism, a turning operation is performed after the clamping mechanism is unclamped.
In general, the clamping mechanism in the rotary table is operated by air pressure or oil pressure. In the clamping mechanism, a subsequent command needs to be given after unclamping is completely performed. For this reason, there is a need to verify whether unclamping is completely performed or not. Whether unclamping is completed or not has been verified using a dedicated sensor, or an unclamping operation has been determined to be completed when a certain period of time passes after a command to perform unclamping is given.
Japanese Patent Application Laid-Open No. 2010-17830 discloses a technology related to a detector that detects a deterioration state of a clamp spring in a mechanism for fixing and releasing a tool by an urging force of the clamp spring with respect to a main spindle in a machine tool.
Japanese Patent Application Laid-Open No. 2007-7735 discloses a technology related to unclamping verification means for determining that a fixing member is in an unclamped state by detecting a position in a turning direction of a turning member by position detection means in a clamping mechanism of a machine tool.
When a conventional dedicated sensor for verification of unclamping is provided, or when an unclamping operation is verified using some kind of a sensor of the technologies disclosed in Japanese Patent Application Laid-Open Nos. 2010-17830 and 2007-7735, mentioned above, the unclamping operation may be reliably performed. However, there is a difficulty in adjusting a sensor, and a separate sensor needs to be provided, and thus there is concern about cost increase.
In a case in which an unclamping operation is determined to be completed when a certain period of time passes after a command to perform unclamping is given, a period of time from when a command to perform unclamping is given from a numerical controller or the like until unclamping is mechanically performed is presumed, and unclamping is regarded to be completed when the period of time passes. Then, a period of time (predetermined time) from when a command to perform unclamping is given until unclamping is completed is set in a timer, and the timer is configured to receive a subsequent command after waiting for the afore-mentioned predetermined time from the instant when the command to perform unclamping is given.
FIG. 4 illustrates timing charts showing an unclamping operation.
In FIG. 4, (a) illustrates an unclamping command which is turned ON at a time t1 and turned OFF at a time t3. A period of time from time t1 to time t3 is set by an unclamp timer. In addition, (c) illustrates that an interlock is configured to be locked between time t1 and time t3 during which the unclamp timer operates. (b) illustrates that a state of a mechanical clamp is changed from a clamped state to an unclamped state by receiving the unclamping command turned ON at time t1, and the change to the unclamped state is completed at a time t2.
Herein, a setting time (from time t1 to time t3) of the unclamp timer is set to be longer than an actual unclamping time (from time t1 to time t2) such that unclamping ends. For this reason, even when unclamping ends (at time t2) after the unclamping time passes, the interlock is in the locked state and thus another command may not be executed while the unclamp timer operates (that is, from time t1 to time t2). For this reason, a subsequent command is executed after the unclamp timer ends (that is, after time t3).
On the contrary, it may take a long time after the unclamping command is received until unclamping mechanically ends due to degradation over time of a component or degradation of grease in a piston part of a clamping mechanism when quality of air is poor or when the clamping mechanism operates by air pressure. In this instance, when it takes an excessively long time until unclamping ends, if original setting of the unclamp timer is unchanged, there is concern that a subsequent command is executed before the mechanical unclamping is completed. In addition, after the unclamping command is given, a rotation operation command is likely to be received. Thus, when a subsequent rotation operation command is started before an actual unclamping operation is completed, there is concern that the clamping mechanism is damaged and thus reliability of the clamping mechanism is degraded.